GROUND WATER IN NORTH-CENTRAL TENNESSEE

104 GROUND WATER IN NORTH-CENTRAL TENNESSEE
(CaCO3) equivalent to all the calcium and magnesium. This quan­
tity is the so-called "total hardness" of the water. It is calculated
by the formula
Total hardness = 2.5 Ca +4.1 Mg
in which all quantities are expressed in parts per million. Hardness
is caused both by the bicarbonates and by the sulphates of calcium
and magnesium. The hardness due to sulphates the so-called "non-
carbonate hardness" or "permanent hardness" may be calculated
from the formula
Noncarbonate hardness = 2.5 Ca+4.1 Mg 0.82 HCO3 .
Water with a total hardness less than 50 parts per million is gen­
erally considered soft, and under most circumstances its treatment to
remove hardness is not justified on the score of economy. Hardness
between 50 and 150 parts per million does not render the water un­
satisfactory for most purposes, but it does increase the consumption
of soap slightly. Hence, it is profitable for laundries and other indus­
tries that use large quantities of soap to soften such a water to remove
calcium and magnesium. Hardness exceeding 150 parts per million
is objectionable in common household uses of water, and if the hard­
ness is 200 or 300 parts per million it is common practice to soften the
water or to install cisterns and rain catches. When an entire munici­
pal supply is softened, the hardness is generally reduced to about 100
parts per million, as the additional improvement from further soften­
ing is not deemed an economy. If the hardness is much more than
100 parts per million, the water must generally be treated for. the
prevention of scale formation before it can be used successfully in
steam boilers. The cost and difficulty of adequate softening for this
purpose are likely to be increased materially if the noncarbonate hard­
ness is large.
Very few of the ground waters of north-central Tennessee contain
less than 50 parts per million total hardness, and many are so hard
that they should be softened to make them suitable for general use.
In some of the waters the noncarbonate hardness also is relatively
large.
There are two processes for softening water in general use the
lime and soda process and the exchange silicate or so-called "zeolite"
or "permutite" process, which has been developed in recent years.
lii the lime and soda process the noncarbonate hardness, if present
in the raw water, is converted to carbonate hardness by the addition
of enough soda (sodium carbonate) to react with the sulphates of
calcium and magnesium to form the bicarbonates of calcium and mag-